Mr Hamon, an ex-education minister who wants to reduce the working week from 35 to 32 hours, tax robots and provide a monthly universal basic income for all, won 58.65 percent of the vote, according to the results announced by organisers based on 60 percent of polling stations having reported results.

Benoit Hamon greets supporters after winning the French Socialist party presidential nomination in ParisCredit:
Francois Mori/AP

"Tonight the Left holds its head up high, looks to the future and wants to win," Mr Hamon told an ecstatic crowd.

Mr Valls, who won a little over 41 per cent of the vote, wished "good luck" to Mr Hamon, who faces an uphill struggle to unite his deeply divided party.

He warned that with the victor of Donald Trump in America and the popularity of far-Right leader Marine Le Pen, the "seeds of political decomposition are present on the Left and Right".

Some pro-Valls MPs have made it clear they will not back Mr Hamon, considered too far to the Left, but the victor insisted that the two camps' positions were "not irreconcilable".

Mr Hamon issued a call to Communist and Green candidates to forge a government coalition with him. Jean-Luc Mélénchon, a Communist firebrand polled to come fourth in the presidential election, has ruled out striking an alliance with the Socialists.

Polls indicate Mr Hamon will likely be eliminated in the first round of the presidential election on April 23.

His victory will boost the chances of maverick candidate Emmanuel Macron - currently the "third man" of the election on a "neither Right nor Left" ticket, commentators predict.

Emmanuel Macron, head of the political movement En Marche !, or Forward !, and candidate for the 2017 French presidential election, attends a political rally in LilleCredit:
Pascal Rossignol/Reuters

Supporters of the business-friendly Mr Valls, who like Mr Macron has steered centrist policies, are likely to give their vote now to the popular ex-banker, who now has a boulevard before him among centrist voters.

The 39-year old is now neck and neck with conservative candidate François Fillon in the first round of the presidential election, according to a Kantar-Sofres-One Point poll out on Sunday. Rocked by allegations that his British wife was paid for a fake job as his parliamentary assistant, Mr Fillon is on 22 per cent with Mr Macron just behind on 21 per cent.

Marine Le Pen, leader of the far-Right Front National, is in pole position in round one on 25 per cent, but is predicted to lose heavily to either contender in the run-off.